Mucosal immune system is widely distributed under mucous membrane of the respiratory tract, gastrointestinal tract and urogenital tract and within the lymphoid tissue at some of the exocrine glands, and is a main place for performing local specific immune function.
Since more than 95% of the body-associated infections occurs in the mucous membranes or happened by invading the body through mucous membranes, so mucous membrane is the largest portal for pathogens to invade the body. Currently, the main infectious diseases those with great hazard for animal life and health as well as those with great difficulties to be controlled, such as influenza, tuberculosis, glanders, and salmonellosis, belong to the mucous membrane invading-through or mucosa-associated diseases, and the resulting mucosal injury and disorders of the mucosal immune function, often become an important mechanism of opportunistic pathogen infection, or even tumorigenesis.
Mucosal immune system is the first immune barrier for the body against invading pathogens, and this independent immune system with distinctive structure and function has a positive meaning for the prevention of pathogen colonization and invasion. Unlike traditional systematic immune system, mucosal immune system comprises the diffuse lymphoid tissue consisted of the large number of immune cells and immune molecules as dispersed in the mucosa epithelium or submucosal lamina propria, or a mucosa-associated lymphoid tissue formed by a single or multiple aggregated lymphoid follicles. More than 50% of the lymphoid tissue of the body and more than 80% of the immune cells concentrated in the mucosal immune system. Furthermore, the antibodies in the mucous secretions are mainly secretory sIgA and slgM antibodies. Mucosal immune system can be divided into two parts by function thereof: inductive site and effective site. Communication between the inductive site and the effective site occurs primarily through homing of lymphocytes. The main function of the mucosal immune system is to recognize and respond to a large number of a wide variety of antigens inhaled or ingested from the mucosal surface. The mucosal immune system not only can reduce the immune response to a large number of harmless antigens or generate tolerance to the same, but also can generate highly efficient humoral immunity and cellular immunity to those harmful antigen or pathogen and perform effective immune rejection or removal.
Mucosal immunity is theoretically the most effective routes of immunization to prevent those pathogens which cause pathogenic role by mucosal infection, because in other ways of immunization, it is difficult to induce significant mucosal immune response by vaccines. However, so far, most of the vaccines which have been approved or are in clinical studies still use the injection route for immunization, only a few of them use route for mucosal immunity. To study the reason, in addition to the difficulties for detection of the local mucosal immune specific antibody, the main difficulties plaguing the development of mucosal immunity vaccine product are, for the role of the body's host defense, during the mucosal immunity it cannot accurately control the level of the antigens into the body as that when in the injection immunization way. After oral or topical administration, antigens reach mucosal surface and then will undergo a dilution by mucous secretion, mucus gel adsorption, protease/nuclease hydrolysis and being cleared by the endometrial barrier. Only very few amount of antigen can go through the mucosa into the body, to play an effective mucosal immune response. In general, the mucosal uptaking rate for those water-soluble antigens and antigens without mucosal function is low, and part of them might cause immune tolerance phenomenon in the intestines. The most effective method to solve the low mucosal uptaking rate is to mimic the features of natural mucosal pathogens and apply effective mucosal adjuvant, as well as enhance the level of combination with mucosal surface of the body (preferably selectively adhered to the M cells). More effective mucosal vaccines are still needed.